1. Field of the Invention
The present invention relates to a system for detecting rotation angles of a diffracting portion, and more particularly to a system including an apparatus and method for detecting rotation angles of a diffracting portion using an absorption cell sealed with a plurality of gases of different kinds having mutually different absorption line wavelengths, as the technology for achieving a correct correspondence of relation between the angle of an optical element of the diffracting portion to be detected in the apparatus for detecting rotation angles of a diffracting portion used in an optical appliance, and the wavelength of the light to be diffracted in a predetermined direction by the diffracting portion.
2. Description of the Related Art
Hitherto, in optical appliances such as an optical spectrum analyzer and a tunable wavelength light source, a diffracting portion composed of an optical element including a diffraction grating is used in order to detect the light to be measured or light of desired wavelength from light of wide band.
The diffraction grating is an element for diffracting the incident light in a direction depending on its wavelength by means of multiple grooves disposed parallel at predetermined intervals on its surface.
In the optical appliances such as an optical spectrum analyzer and a tunable wavelength light source, it is designed to vary the wavelength of the light diffracted in a desired direction, by varying the angle of the diffraction grating of the diffracting portion according to the incident light, or varying the angle of a reflector for reflecting the diffracted light from the diffraction grating to the diffraction grating with respect to the diffraction grating.
Therefore, an optical appliance having such a diffracting portion comprises a driving device for rotating the diffraction grating or reflector, and an angle detector for detecting its angle, and the corresponding relation between the angle detected by the angle detector and the wavelength of the light diffracted in a predetermined direction (hereinafter called angle-wavelength relation) is stored preliminarily.
In such optical appliances, for example, when a specific wavelength is designated, the diffraction grating or reflector for composing the diffracting portion is set at an angle corresponding to the designated wavelength so that the light of the designated wavelength may be diffracted in a predetermined direction, or the wavelength of the light diffracted in a predetermined direction is determined from the angle detected by the angle detector.
In this case, however, the characteristic of the optical system including the diffracting portion changes slightly according to environmental conditions, such as ambient temperature, humidity or atmospheric pressure, and this characteristic varies the angle-wavelength relation.
To solve this problem, the present applicants etc. disclosed an apparatus for detecting rotation angles of a diffracting portion intended to determine the angle-wavelength relation by using an absorption cell extremely small in change in wavelength due to changes in environmental conditions in U.S. Pat. No. 5,828,061.
The absorption cell used in this apparatus for detecting rotation angles of a diffracting portion is sealed with a single specific gas, and it absorbs the light of a wavelength (absorption line wavelength) determined by this gas from the incident light, and emits the light of the absorption wavelength.
This absorption line wavelength is a known value determined by the kind of the single gas sealed inside, and is hardly changed by changes in environmental conditions such as ambient temperature, humidity or atmospheric pressure, and the precision of the absorption line wavelength is extremely high.
Accordingly, this apparatus for detecting rotation angles of a diffracting portion is capable of determining accurately the angle-wavelength relation of the diffraction grating in the present environments by determining the angle detected by the angle detector when the diffraction grating diffracts the light of this absorption line wavelength in a predetermined direction.
In this case, the absorption lines of the specific single gas sealed in the absorption cell are concentrated in a relatively narrow wavelength region.
Accordingly, this apparatus for detecting rotation angles of a diffracting portion can accurately determine the angle-wavelength relation near the wavelength region, but cannot accurately determine the angle-wavelength relation in a wavelength region far apart from this wavelength region.
Therefore, in optical appliances required to vary the wavelength of measuring light or output light in a wide range such as the optical spectrum analyzer or the tunable wavelength light source, high wavelength precision cannot be maintained in a full wide range.
To solve this problem, simply, it may be considered to determine each specific rotation angle by selectively changing over a plurality of absorption cells sealed with a single gas of different kinds, but in such a configuration the rotation angle between specific rotation angles cannot be determined, and the structure of the entire apparatus becomes complicated.
It is hence an object of the invention to solve these problems and present an apparatus and method for detecting rotation angles of a diffracting portion capable of accurately determining the angle-wavelength relation in a wide wavelength region in a simple configuration.
In order to achieve the above-described object, according to a first aspect of the present invention, there is provided an apparatus for detecting rotation angles of a diffracting portion comprising:
a diffracting portion which is composed of optical elements including a predetermined optical element and diffracts light;
a drive unit which rotates the predetermined optical element of the diffracting portion;
an angle detector which detects the rotation angle of the predetermined optical element;
a reference wavelength light source which includes a light source and an absorption cell and emits the reference light of a specific wavelength determined depending on the absorption cell toward the diffracting portion;
a reference photodetector which receives a diffracted light emitted in a predetermined direction from the diffracting portion receiving the reference light from the reference wavelength light source, and converts the diffracted light into an electric signal; and
a signal processor which receives a rotation angle to be issued by the angle detector when detecting a predetermined value of the electric signal from the reference photodetector by rotating the predetermined optical element by the drive unit, and determines the predetermined rotation angle corresponding to the specific wavelength,
wherein the absorption cell is sealed with a plurality of gases of different kinds having mutually different in the wavelength region in which absorption lines exist, and
the signal processor is designed to determine plural specific rotation angles corresponding to the specific wavelength determined depending on the plurality of gases of different kinds sealed in the absorption cell, and also determine the rotation angle between plural specific rotation angles by predetermined complementary calculation including linear complement.
In order to achieve the above-described object, according to a second aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the first aspect, wherein the plurality of gases of different kinds sealed in the absorption cell are at least two gases selected from carbon dioxide (CO2) of which absorption line wavelengths are concentrated around 1.43 xcexcm, acetylene gas (C2H2) of which absorption line wavelengths are concentrated around 1.53 xcexcm, and methane gas (CH4) of which absorption line wavelengths are concentrated around 1.65 xcexcm.
In order to achieve the above-described object, according to a third aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the first aspect, wherein the plurality of gases of different kinds sealed in the absorption cell are a first gas such as acetylene gas (C2H2) of which absorption line wavelengths are concentrated around 1.53 xcexcm and a second gas such as methane gas (CH4) of which absorption line wavelengths are concentrated around 1.65 xcexcm, and
the signal processor determines first and second specific rotation angles corresponding to first and second specific wavelengths determined depending on the first and second gases sealed in the absorption cell, and determines the rotation angle between plural specific rotation angles by predetermined complementary calculation including linear complement, and therefore the apparatus for detecting rotation angles of a diffracting portion is designed to cover the wavelength region of the light in part of S band (1.46 xcexcm to 1.53 xcexcm) and C band (1.53 xcexcm to 1.565 xcexcm) on the basis of the specific wavelength determined depending on the first gas, and L band (1.565 xcexcm to 1.625 xcexcm) on the basis of the specific wavelength determined depending on the second gas.
In order to achieve the above-described object, according to a fourth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the first aspect, wherein the light emitted from the absorption cell in the reference wavelength light source has a plurality of absorption lines in each wavelength region, and
the signal processor calculates the rotation angle determining whether or not the light received by the reference photodetector corresponds to the absorption line of which wavelength from the level change of the diffracted light issued from the diffracting portion, by making use of the characteristic that the plural absorption lines of each wavelength region have mutually different absorption rates in each absorption line.
In order to achieve the above-described object, according to a fifth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the fifth aspect, further comprising:
a rotation angle setting unit which has angle-wavelength data and issues to the drive unit angle data for driving so that the predetermined optical element of the diffracting portion may be set at a rotation angle corresponding to the wavelength value, and
a data correcting unit which corrects the angle-wavelength data possessed in the rotation angle setting unit based on the plural specific rotation angles corresponding to the plural specific wavelengths in each wavelength region.
In order to achieve the above-described object, according to a sixth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the first aspect, wherein the apparatus for detecting rotation angles of a diffracting portion is used for analyzing the spectrum of the light to be measured, and the signal processor has a feeding unit which feeds correction data from the data correcting means to the drive unit in order to correct the angle of the predetermined optical element of the diffracting portion prior to spectrum analysis of the light to be measured.
In order to achieve the above-described object, according to a seventh aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the sixth aspect, further comprising:
a measuring photodetector which receives the diffracted light of the light to be measured while the predetermined optical element of the diffracting portion is rotating according to the set wavelength value, and converts the diffracted light into an electric signal, and
a spectrum processor which issues the electric signal issued from the measuring photodetector by relating to the set wavelength value.
In order to achieve the above-described object, according to an eighth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the sixth aspect, further comprising:
a measuring photodetector which receives the diffracted light of the light to be measured while the predetermined optical element of the diffracting portion is rotating according to the set wavelength value, and converts the diffracted light into an electric signal, and
a spectrum processor which determines an electric signal at the set wavelength value based on the angle-wavelength data corrected by the data correcting unit, from the electric signal issued by the measuring photodetector, and issues by relating the set wavelength value with the electric signal.
In order to achieve the above-described object, according to a ninth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the eighth aspect, further comprising a changing unit which changes over between the reference light from the reference wavelength light source and the light to be measured.
In order to achieve the above-described object, according to a tenth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the eighth aspect, wherein the reference photodetector and measuring photodetector are realized by one common unit.
In order to achieve the above-described object, according to an eleventh aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the fourth, wherein the apparatus for detecting rotation angles of a diffracting portion is used in a tunable wavelength light source of external resonance type for reflecting the light between the diffracting portion and the light source, and issuing the resonating variable wavelength light, and
the signal processor includes a feeding unit which feeds correction data from the data correcting means to the drive unit in order to correct the rotation angle of the predetermined optical element of the diffracting portion prior to outputting the tunable wavelength light from the tunable wavelength light source of external resonance type.
In order to achieve the above-described object, according to a twelfth aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the eleventh aspect, further comprising a changing unit which changes over between the reference wavelength light source and the main light source.
In order to achieve the above-described object, according to a thirteenth aspect of the present invention, there is provided a method of detecting rotation angles of a diffracting portion comprising:
emitting a reference light of a specific wavelength determined depending on an absorption cell from a reference wavelength light source including a light source and the absorption cell;
diffracting the reference light emitted from the reference wavelength light source, by a diffracting portion composed of optical elements including a predetermined optical element;
rotating the predetermined optical element of the diffracting portion by a drive unit;
detecting the rotation angle of the predetermined optical element by an angle detector;
receiving a diffracted light emitted in a predetermined direction from the diffracting portion receiving the reference light from the reference wavelength light source, and converting the diffracted light into an electric signal by a reference photodetector; and
receiving a rotation angle issued by the angle detector when detecting a predetermined value of electric signal from the reference photodetector by rotating the predetermined optical element by the drive unit, and determining the predetermined rotation angle corresponding to the specific wavelength by a signal processor,
wherein the absorption cell is sealed with a plurality of gases of different kinds having mutually different in the wavelength region in which absorption lines exist, and
the signal processor determines plural specific rotation angles corresponding to the specific wavelength determined depending on the plurality of gases of different kinds sealed in the absorption cell, and also determines the rotation angle between plural specific rotation angles by predetermined complementary calculation including linear complement.
In order to achieve the above-described object, according to a fourteenth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the thirteenth aspect, wherein the plurality of gases of different kinds sealed in the absorption cell are at least two gases selected from carbon dioxide (CO2) of which absorption line wavelengths are concentrated around 1.43 xcexcm, acetylene gas (C2H2) of which absorption line wavelengths are concentrated around 1.53 xcexcm, and methane gas (CH4) of which absorption line wavelengths are concentrated around 1.65 xcexcm.
In order to achieve the above-described object, according to a fifteenth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the thirteenth aspect, wherein the plurality of gases of different kinds sealed in the absorption cell are a first gas such as acetylene gas (C2H2) of which absorption line wavelengths are concentrated around 1.53 xcexcm and a second gas such as methane gas (CH4) of which absorption line wavelengths are concentrated around 1.65 xcexcm, and
the signal processor determines first and second specific rotation angles corresponding to first and second specific wavelengths determined depending on the first and second gases sealed in the absorption cell, and determines the rotation angle between plural specific rotation angles by predetermined complementary calculation including linear complement, and therefore the apparatus for detecting rotation angles of a diffracting portion is designed to cover the wavelength region of the light in part of S band (1.46 xcexcm to 1.53 xcexcm) and C band (1.53 xcexcm to 1.565 xcexcm) on the basis of the specific wavelength determined depending on the first gas, and L band (1.565 xcexcm to 1.625 xcexcm) on the basis of the specific wavelength determined depending on the second gas.
In order to achieve the above-described object, according to a sixteenth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the thirteenth aspect, wherein the light emitted from the absorption cell in the reference wavelength light source has a plurality of absorption lines in each wavelength region, and
the signal processor calculates the rotation angle determining whether or not the light received by the reference photodetector corresponds to the absorption line of which wavelength from the level change of the diffracted light issued from the diffracting portion, by making use of the characteristic that the plural absorption lines of each wavelength region have mutually different absorption rates in each absorption line.
In order to achieve the above-described object, according to a seventeenth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the thirteenth aspect, further comprising:
receiving a set wavelength value and issuing to the drive unit angle data to drive so that the predetermined optical element of the diffracting portion may be set at a rotation angle corresponding to the wavelength value, by a rotation angle setting unit having angle-wavelength data; and
correcting the angle-wavelength data possessed in the rotation angle setting unit based on the plural specific rotation angles corresponding to the plural specific wavelengths in each wavelength region, by a data correcting unit.
In order to achieve the above-described object, according to an eighteenth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to seventeenth aspect, further comprising:
feeding correction data from the data correcting unit to the drive unit in order to correct the angle of the predetermined optical element of the diffracting portion prior to spectrum analysis of the light to be measured, being applied in analysis of spectrum of the light to be measured.
In order to achieve the above-described object, according to a nineteenth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the eighteenth aspect, further comprising:
receiving the diffracted light of the light to be measured while the predetermined optical element of the diffracting portion is rotating according to the set wavelength value, and converting the diffracted light into an electric signal, by a measuring photodetector; and
issuing the electric signal issued from the measuring photodetector by relating to the set wavelength value, by a spectrum processor.
In order to achieve the above-described object, according to a twentieth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the eighteenth aspect, further comprising:
receiving the diffracted light of the light to be measured while the predetermined optical element of the diffracting portion is rotating according to the set wavelength value, and converting the diffracted light into an electric signal, by a measuring photodetector; and
determining an electric signal at the set wavelength value based on the angle-wavelength data corrected by the data correcting unit, from the electric signal issued by the measuring photodetector, and issuing by relating the set wavelength value with the electric signal, by a spectrum processor.
In order to achieve the above-described object, according to a twenty first aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the nineteenth aspect, further comprising:
changing over between the light from the reference wavelength light source and the light to be measured.
In order to achieve the above-described object, according to a twenty second aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the nineteenth aspect, further comprising realizing the reference photodetector and measuring photodetector by one common unit.
In order to achieve the above-described object, according to a twenty third aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the seventeenth aspect, further comprising:
being used in a tunable wavelength light source of external resonance type which reflects the light between the diffracting portion and the light source, and issues the resonating tunable wavelength light;
feeding correction data from the data correcting unit to the drive unit in order to correct the rotation angle of the predetermined optical element of the diffracting portion prior to output the tunable wavelength light from the tunable wavelength light source of external resonance type.
In order to achieve the above-described object, according to a twenty fourth aspect of the present invention, there is provided the method of detecting rotation angles of a diffracting portion according to the twenty third aspect, further comprising:
changing over between the reference wavelength light source and the main light source.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.